Breakthrough in fixing gene defects

Doctors have treated a life-threatening blood disease by repairing flaws in the genetic code of a living animal, the first time such an ambitious feat has been achieved.

The work raises the prospect of powerful new therapies that can target and repair the genetic defects behind a wide range of human diseases that cannot be tackled with modern medicines.

The new technique, called genome editing, holds particular promise for a group of illnesses that run in families and are caused by faults in genes that underpin the healthy working of the immune system, bone marrow and liver.

To demonstrate the therapy, researchers treated mice that were bred to develop haemophilia B, an inherited bleeding disorder that destroys the body’s ability to form blood clots.

Normally, when the body suffers a cut or graze, proteins called clotting factors combine with platelet cells in the blood to make it sticky and form a clot that stops any bleeding.

But people born with haemophilia B carry a defect in a gene that makes clotting proteins, leaving them vulnerable to excessive bleeding, even when they have not sustained an injury.

About one in 30,000 boys are born with haemophilia B which, at its most severe, requires patients to have frequent infusions of blood clotting factors to prevent spontaneous haemorrhages.

In a report in the journal Nature, a team led by Katherine High at the Children’s Hospital of Philadelphia describes how genome editing reversed haemophilia B in mice, restoring their blood clotting times to near-normal without causing any apparent side-effects.